1. Field of Invention
The electric control and supply system comprises a supply and control assembly at a first location and a control and actuating assembly at a remote location associated with a remote device. An umbilical extends between and connects the supply and control assembly with the control and actuating assembly for supplying direct voltage to the control and actuating assembly. The electric control and supply system may be used, for example, in the production of oil and gas and may be used either with land based wells or offshore wells. With offshore wells, the supply and control assembly is disposed on a platform or vessel at the sea surface and the control and actuating assembly is located at a remote location below the sea surface such as at the sea floor. The umbilical extends subsea from the supply and control assembly supplying direct voltage to the remote subsea control and actuating assembly. The subsea control and actuating assembly is connected to various electrical devices, such as motors, electrical actuators and similar equipment via appropriate connecting lines.
2. Background of the Art
Typically, subsea tools (e.g., controls systems and actuators) are hydraulically controlled and actuated. However, hydraulic supply lines are large and expensive. Further, hydraulic equipment, such as pumps at the surface, are large and take up a significant amount of space on the platform or vessel. One way to solve the problems presented by hydraulic equipment is to implement electrically powered subsea tools. Therefore, electrical control and power supply systems for subsea tools are needed.
Prior art electrical control and supply systems include an energy supply system at the sea surface, which transmits alternating voltage through a subsea cable to the sea floor. The amplitude and frequency of the alternating voltage is selected such that, for example, the subsea tools connected to the end of the subsea cable receive a suitable supply voltage for their operation. Each subsea tool is connected to a separate subsea cable. Furthermore, data transmission between the surface and the sea floor occurs via separate subsea cables.
Referring to FIG. 1(a), there is shown a prior art control and supply system 1 having a voltage supply and control device 3 with appropriate voltage source and multiplexer device 7 arranged above the surface of the sea 4. The voltage supply 3 transmits alternating voltage directly, via a subsea cable 5, to a control and actuating device 6 arranged below sea level. The control and actuating device 6 is connected via connecting lines 8 to appropriate electrical devices 2 or electrical units 9. An electrical unit 9 may be formed by a group of electrical devices 2, which, for example, are arranged in the form of a tree structure and are controlled and actuated on a common basis.
A data cable 10 is provided for the transmission of data and control signals between the voltage supply and control device 3 and the control and actuating device 6. The data cable 10 is preferably composed of coaxial conductors.
Normally, an alternating voltage of a maximum of 600 VAC is transmitted along the subsea cable 5. For the supply of the appropriate electrical devices with 240 VAC and appropriate power, cross-sectional areas of at least 175 mm2 for appropriate conductors are required in the subsea cable having a length, for example, of 50 km.
The control and actuation device 6 includes at least one motor actuation device 11 and a control system 12. The various motors, as electrical devices 2, can be used subsea for the actuation of valves, BOPs (blow-out preventers) and similar equipment used for the production of oil or gas at the sea floor.
One disadvantage with prior art control and supply systems, such as shown in FIG. 1(a), is that a costly subsea cable is necessary. For example, to supply a subsea electrical device with 240 VAC via a subsea cable that extends 30 to 50 km from the surface down to the subsea electrical device, the subsea cable must have a cross-sectional area of 100 to 200 mm2. In addition, data lines are required, such that the subsea cable must have a substantial diameter, and thus be very costly.
In the above example, it has been assumed that 240 VAC is sufficient for the subsea electrical devices. However, it has now been found that higher voltages are required, for example, in order to be able to actuate certain subsea electrical devices, such as servomotors requiring greater power, for example, to close valves in the production of oil and gas in a maximum time period of one minute. Where such electrical devices must be supplied with a greater voltage, the cross-sectional area of the subsea cable increases still further.
In addition, it has been found in practice that on starting a servomotor as an electrical device and in particular for servomotors requiring greater power, even with a slow starting process, a return signal is transmitted via a subsea cable to the supply and control device at the surface indicating the starting process of the servomotor as a short circuit at the end of the cable. This leads to the switching off of any systems automatically protected against short circuit.
Furthermore, with the previously described prior art control and supply system, the overall system only has an output power efficiency of 27%.
Another known control and supply system is shown in FIG. 1(b) with the transmission of alternating voltage along the subsea cable 5. In this case, however, a voltage of a maximum of 10,000 VAC is transmitted which is reduced, before the control and actuation device 6, by a suitable transformer 13 to the voltage values required for the electrical devices. Also, with this prior art system, a separate data conductor 10 is provided as a coaxial cable or similar cable. The control and actuating device 6 according to FIG. 1(b) requires expensive power capacitors 14 in order to smooth the reduced alternating voltage appropriately. In addition, with this prior art system, as with the system according to FIG. 1(a), power factor correction devices are needed to lower the apparent power of the system to obtain an adequate efficiency for the overall system. Such correction devices are very complex and normally quite expensive and consist of capacitors or similar devices.
With the prior art system according to FIG. 1(b) and for appropriate voltage values and powers for the electrical devices on the sea floor, conductor cross-sectional areas in the subsea cable of, for example, at least 75 mm2 arise for a length of 50 km or with power factor correction at least a cross-sectional area of 26 mm2 for a 50 km length.
However, even with the complete expansion of the previously mentioned prior art system, the efficiency normally is less than 70% and the cross-sectional areas for a conductor in the subsea cable are about 16 or 26 mm2 for a length of 30 km or 50 km, respectively.
Converting devices have been used to convert a high voltage (DC or AC) to a lower voltage (DC or AC). If a high voltage is present on the input side, a corresponding conversion into another voltage is difficult as a rule because corresponding components of the converting device do not show a sufficiently high breakdown strength. Moreover, in the case of a high power to be transmitted, the heat developed in the converting device may be considerable even if the power loss is only 10 or 20%. To be able to discharge the power loss converted into heat, corresponding cooling means must be provided. This makes the converting device more expensive and also larger due to the additional cooling means. Components having dielectric strengths of more than 1000V, e.g. 3000 or 6000V, are, however, not available or they can hardly be realized technically. If such a converter is nevertheless suitable for such high DC voltages, the whole system will collapse if the converter fails to operate. In addition, even if the efficiency is comparatively high, the converting device will have a dissipation power that produces a substantial amount of heat comparatively locally. This amount of heat may destroy certain components of the converting device. In order to avoid such destruction, complicated cooling systems are required which entail high costs.
The present invention overcomes the deficiencies of the prior art.